Various commercial articles have been manufactured from porous materials having fibrillar microstructures including various implantable medical devices. Examples of these materials may include porous expanded polytetrafluoroethylene (ePTFE) and stretched polyethylenes and polypropylenes. Articles fabricated from these porous fibrillated materials include fabrics, battery membranes, various filters, electrical insulation and various medical devices including vascular grafts, tissue repair patches, sutures and stent-grafts. Porous materials used to manufacture articles such as these can take the form of, for example, sheets, thin films and tubes. It is known that nominal physical dimensions of some of these articles may be changed (reduced) by applying compressive forces to these materials, particularly in a direction substantially parallel to the predominant direction of the fibrils. Compressive forces applied to these materials result in a decrease in porosity (i.e., an increase in bulk density; a decrease in the volume of void space) as the material is moved into the available void space due to the compressive force. Compressive forces applied to these fibrillar materials in directions substantially parallel to the general directional orientation of the fibrils will result in bending of the previously substantially straight fibrils. The compressed material may then be heated for suitable time and temperature so that the bent form of the fibrils becomes permanent. Fibrillar materials so processed to have these formed bent fibrils are typically extensible by applied tensile force in a direction generally parallel to the directional orientation of the fibrils. Upon release of the tensile force, these materials will typically recover most or all of the extended length. The use of compression to form bent fibrils in fibrillated materials is taught in U.S. Pat. No. 5,308,664 to House and Myers.
It is also known to apply coatings of various materials to fibrillated polymeric substrates for a variety of purposes. Thermoplastic coatings on such substrates are sometimes used as adhesives for bonding together different components of an article. A thermoplastic coating of fluorinated ethylene propylene (FEP) applied to ePTFE, for example, is taught by U.S. Pat. No. 5,735,892 to Myers et al.
For some applications, however, it may be advantageous for a product to remain at the partially or fully extended dimension following release of the extending tensile force. A fracturable coating applied to a porous substrate material having a microstructure of bent fibrils may be used to create an article that can be, during normal use, permanently increased in at least one dimension by the application of a tensile force.